1. Field of the Invention
The present invention relates to an apparatus for processing signals in a mobile communication terminal and a method for controlling the same, and more particularly to an apparatus for processing RF (Radio Frequency) transmission signals in a mobile communication terminal and a method for controlling the same.
2. Description of the Related Art
Typically, since a mobile terminal and a base station are spaced apart from each other at a predetermined distance in a mobile communication system, they perform a transmission/reception operation on a predetermined frequency band. That is, data transmission between the mobile terminal and the base station is performed using an RF signal of a predetermined frequency band. Noise is a greater problem in such a wireless communication system than a wired communication system because the data transmission is wirelessly performed. The noise may occur due to limitations of a channel environment or a transmitter.
The noise generated by the channel environment will be discussed below. The noise of the channel environment may be generated by interference between two similar frequencies in a radio channel, or may be generated by a topographical object, etc. The noise generated in the radio channel will be discussed below. Firstly, in case of a noise generated by the interference between two similar frequencies, a frequency band of data to be transmitted is raised to a predetermined frequency band and then the data is transmitted via the predetermined frequency band. Therefore, in the case where a signal having a frequency similar to the raised frequency band is adjacent to the signal of the data to be transmitted, signal interference may occur between the two signals under the condition that two wireless terminals using two similar frequencies are adjacent to each other. Next, as for the noise generated by a topographical object, data to be wirelessly transmitted is raised to a predetermined frequency band and then the data is transmitted via the predetermined frequency band. Since the data has a high frequency, noise may be caused by refraction and diffraction phenomena, and noise may be also caused by a time delay owing to a difference between transmission paths.
In the case of radio data transmission, besides the noise generated in the radio channel, noise is generated in a transmission device (i.e., a transmitter). The noise generated in the transmitter will be hereinafter described below. The noise generated in the transmitter occurs during signal step-up conversion or transmission amplification. Comparing the noise generated in the transmitter with the other noise generated in the other bands (except for a band of the transmitter in the case of a spreading spectrum system generally used), the noise generated in the transmitter comprises 80% of an overall noise factor. So, a mobile communication system makes all possible effort to remove such noise generated in the transmitter.
A detailed block diagram of such transmitter generally used for transmitting RF signals will hereinafter be described with reference to FIGS. 1 and 2. FIG. 1 is a block diagram of an RF transmitter for reducing noise in a mobile communication terminal.
Referring to FIG. 1, a baseband processor 10 converts baseband signals, i.e., data to be transmitted, into symbols. Such symbols generated from the baseband processor 10 are amplified in a variable amplifier 12, filtered in a BPF (Band Pass Filter) 14, and then input to a mixer 16. The mixer 16 converts the symbols of a baseband into RF signals of a predetermined band, that is, performs a step-up conversion from the baseband symbols to the RF signals of a predetermined band. The step-up converted signals, i.e., RF signals from the mixer 16, are filtered in a BPF 18, amplified in a variable amplifier 20, and then input to a fixed attenuator 22. The fixed attenuator 22 attenuates the received signals with a predetermined fixed attenuation value. The attenuated signals from the fixed attenuator 22 are input to a BPF 24, filtered in the BPF 24, and then input to a variable amplifier 26. Then, the variable amplifier 26 amplifies the received signals to make a predetermined gain value therein, and transmits the amplified signals to a power amplifier 28. The power amplifier 28 amplifies power levels of the amplified signals, and then transmits the resultant signals to an antenna.
The aforementioned circuit shown in FIG. 1 attempts to remove the noise generated in an RF transmitter by means of the fixed attenuator 22. Likewise, the fixed attenuator 22 can perform a sequential power adjustment in such a way that it satisfies an ACPR (Adjacent Channel Power Rejection Ratio) required by an RF specification (also called a spec) of a mobile communication system. However, the RF transmitter of FIG. 1 has a disadvantage in that it degrades a level of input power in accordance with an ACPR of a maximum output power because the fixed attenuator 22's value is always fixed to a predetermined value. Also, the RF transmitter of FIG. 1 has a further disadvantage in that it has a high power variation at a low temperature.
FIG. 2 is a view illustrating a block diagram of another RF transmitter for reducing noise in a mobile communication terminal. Comparing the RF transmitter of FIG. 1 with the RF transmitter of FIG. 2, the RF transmitter of FIG. 2 adopts a drive amplifier 50 whereas the RF transmitter of FIG. 1 adopts a fixed attenuator 22. Except for this difference between them, the components of RF transmitter of FIG. 2 are the same as the RF transmitter of FIG. 1. The drive amplifier 50 adopts a step gain adjustment method. The RF transmitter of FIG. 2 uses the drive amplifier 50, thereby using less power, improving its temperature characteristic, and simplifying a control circuit. The drive amplifier 50 has an unavoidable discontinuity in the case of adjusting a power level because it adopts the step gain adjustment method. As a result, in the case of measuring an RF signal such as a signal “open_loop_time_response”, the output power may suddenly rise or spike. This may result in exceeding the power range required by an RF specification of a mobile communication terminal.